icing

icing

[′ī·siŋ]

(hydrology)

Any deposit or coating of ice on an object, caused by the impingement and freezing of liquid (usually supercooled) hydrometeors.

A mass or sheet of ice formed on the ground surface during the winter by successive freezing of sheets of water that may seep from the ground, from a river, or from a spring. Also known as flood icing; flooding ice.

Icing

a dangerous phenomenon that causes deterioration of the aerodynamic characteristics, flight performance, stability, and controllability of an aircraft; it also increases frontal drag. It can interfere with the operation of engines, navigational instruments, and radio equipment and can lead to a crash.

A distinction is made between in-flight icing and ground icing. In-flight icing occurs mainly when an aircraft strikes supercooled water drops of clouds or precipitation, which then freeze. The thickness of the ice layer on some parts of the aircraft may be 10 cm or more. For in-flight icing, the manner of ice deposition, the shape and structure of the ice, and the rate of formation depend on the size and concentration of the drops, as well as on the heat-exchange processes that take place on the surface that is icing up. The smaller the drops and the lower the flight speed, the more readily the drops are entrained by the airstream and, consequently, the smaller the proportion of drops that will strike the aircraft. The higher the air temperature and the airstream velocity and the larger the drop size and the greater the concentration, the more optically uniform and transparent the ice layer and the more uneven and hummocky its surface. This leads to significant deterioration of the air flow around the supporting surfaces (wings) of the aircraft. The optical uniformity of such a layer results from the freezing of ice under a thin film of water, which does not freeze before new drops arrive. At low temperatures and drop concentrations, a smooth and, consequently, less dangerous ice layer forms; because of the presence of air inclusions, the ice is opaque and often has a milky white color.

As the flight speed increases, the heating of aircraft surfaces also increases, leading to increased evaporation of ice from the surfaces. At supersonic speeds the heating and evaporation are great, and icing of surfaces becomes improbable; however, even in this case there is a dangerous possibility of icing of engines, which can occur in crystalline clouds and even in cloudless skies. As air is sucked into the engines, adiabatic expansion may result in its supersaturation with water vapor, the formation of drops, and icing of the intake parts of the engine. Provisions for preventing icing in flight may be passive (correct choice of air route and flight altitude) or active (heating or deicing). The most widely used anti-icing device is an electric deicer with provisions for cyclic heating.

Icing of aircraft parked on the ground can occur if supercooled rain or wet snow freezes on the aircraft’s surfaces. For this reason, parked aircraft are usually protected by jackets.

icing

The accretion of ice on aircraft wings and other parts caused by supercooled water droplets in the atmosphere, cooling by the evaporation of a wetted surface or isotropic expansion, or supercooled fuel in the wings in high humidity conditions. See also glaze.

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